What will happen if I don't have the periodontitis treated?

The final result of periodontal disease left untreated is teeth falling, because of loss of bone structure, which suppports them in the mouth. The loss of natural dentition has detrimental consequenses to a person's  quality of life. Without teeth you cannot enjoy food, you cannot speak properly, you cannot smile, you cannot socialize. You can argue that fact  by saying that nobody is left without teeth nowdays. There are solutions for replacing the natural dentition with either a removable denture or dental implants. This is definetely true, but you should keep in mind that in case you lost your teeth due to periodontitis, there will be bone loss and that will create problems in the stability of the denture. In case you decide to go with implants, after loosing your teeth of  periodontitis, you should know that there will probably be need for bone grafing which will raise the cost and the time of therapy as well as the patient's discomfort. Then remember that if you lost teeth due to periodontitis, there is a possibility that your implants will be affected by  the same disease (it is called periimplantitis). So not seeking for periodontal therapy, if needed makes no sense at all.

Besides the effects of periodontal disease in the mouth, there are also side effects iin one's general health. Periodontal disease has been associated with diabetes in a two way relationship:  Diabetes can increase the chance of having  periodontitis but also periodontal disease can make it more difficult to control blood sugar in diabetic patients. Also periodontitis has been associated with increased risk of heart disease and heart attack and increased risk of a preterm low birth weight baby in case of periodontitis during pregnancy. So it is true when the Ancient Greek used to say that the health starts from the mouth.